Color photographic materials and methods commonly employ silver halide emulsions with silver chloride emulsions being particulary suitable in many applications. In photofinishing processes which use photosensitive paper to produce color prints, it is generally desirable to shorten the processing time. One way to shorten the processing time is to accelerate the development rate by increasing chloride content of the silver chloride emulsions, i.e., the higher the chloride content, the faster the development rate. However, it is often difficult to obtain high, invariant photosensitivity when higher chloride contents are employed. Typically, silver chloride emulsions exhibit increased fogging characteristics and emulsion sensitivity changes owing to high temperatures and/or humidity conditions which may be incurred during storage. The increases in fogging characteristics and the emulsion sensitivity changes may vary from layer to layer in a photographic material, thus resulting in increased color imbalance and a loss of quality in the printed material. These disadvantageous effects are increased with higher silver chloride contents.
Various attempts have been made to reduce fog formation during storage of photographic materials by the addition of stabilizing agents or deterioration inhibiting agents to the silver halide emulsions. For example, U.S. Pat. Nos. 2,440,110, 2,948,614, 3,043,696, 3,057,725, 3,226,232, 3,397,986, 3,447,925 and 3,761,277 and U.S. Publication No. T866,036 disclose the addition of various organic disulfide compounds to silver halide emulsions in order to reduce the increase in fogging characteristics in color photographic materials. However, disulfides which inhibit fog formation can also reduce emulsion sensitivity, thereby resulting in a loss in photographic speed. In fact, U.S. Pat. Nos. 3,184,313 and 3,403,025 disclose the use of disulfides to desensitize silver halide emulsions to visible radiation. Thus, a need exists for stabilizing silver halide emulsions against fogging without reducing the sensitivity of the emulsions, thereby preventing a loss in photographic speed.
Another disadvantage which is caused by the use of a high silver chloride content in color print paper is an increased sensitivity to high temperatures during exposure. The increased sensitivity can also contribute to changes in photographic speed. For example, when the temperature upon exposure rises, i.e., owing to heat from a lamp or the like during printing, the print density changes if the printing conditions are not adjusted to compensate for the rise in temperature, thereby making it very difficult if not impossible to obtain excellent quality prints. Additionally, an increase in temperature during exposure of the paper often results in a selective increase in speed in one layer, for example the cyan layer, over another light sensitive layer, such as the magenta layer, thereby resulting in an improper color balance in the color print. The photofinishing process must then be adjusted to compensate for this density fluctuation. As will be apparent, operating efficiency is decreased.
European Reference No. 367,227 discloses that the heat sensitivity of high content silver chloride color paper material may be reduced by employing certain spectral sensitizing dyes in combination with mercapto azoles. However, these dye structures do not appear to be entirely satisfactory in terms of minimizing thermal sensitivity while maintaining optimal sensitization efficiency. European Reference No. 325,235 discloses the use of iron ion donating compounds to reduce the change in sensitivity owing to exposure at elevated temperatures. Despite these attempts to reduce the thermal sensitivity of high content silver chloride emulsions during exposure, a need still exists for overcoming this problem.